1. Field of the Invention
The present invention relates to an image pickup lens for forming an image of an object on a solid imaging element such as a CCD sensor or a C-MOS sensor adopted in small-sized imaging devices. More specifically, the present invention relates to an image pickup lens composed of five lenses, which is built into an imaging device mounted on portable terminals such as cellular phones and smartphones, PDAs (Personal Digital Assistances), and game machines or information terminals such as personal computers and the like, where downsizing and thinning are pursued.
2. Description of the Related Art
Recently, the market of portable terminals having imaging devices has been expanding increasingly. Most portable terminals are equipped with a camera function. Currently, the majority of such camera functions has a large number of pixels comparable to that of digital cameras. Along with the increasing demands for thinning of portable terminals for reasons such as user-friendliness and design, demands for downsizing and thinning of the imaging devices built therein are also becoming severe. Further, as for the image pickup lens mounted on imaging devices adopting such imaging elements having a large number of pixels, there are demands for a lens system having even higher resolution, which is downsized, thinned and brighter (that is, with a small F-number). At the same time, there is also a strong demand for an imaging lens corresponding to a wide angle of field, which enables the camera to take an image of an object in a wide area.
Many image pickup lenses having a four-lens configuration have been proposed as the image pickup lens answering to such trend of downsizing, thinning and enhanced performance. However, in order to cope with further downsizing and increase in the number of pixels, many image pickup lenses having a five-lens configuration, capable of realizing a higher performance than the four-lens configuration, have been proposed. The present invention corresponds to such five-lens configuration.
For example, Japanese Patent Laid-Open No. 2009-294528 (Patent Document 1) discloses an image pickup lens having a five-lens configuration composed of, in order from an object side, a first lens having a positive power with an object side surface formed to have a convex surface, a stop, a second lens having a meniscus shape near an optical axis, a third lens having an image side surface formed to have a convex shape near the optical axis, a fourth lens having both sides thereof formed as aspherical surfaces and where a circumference portion of an image side surface is formed to have a convex shape, and a fifth lens having both sides thereof formed as aspherical surfaces and where a circumference portion of an image side surface is formed to have a convex shape, wherein only one of the second to fifth lenses is a negative lens having an Abbe number of 30 or smaller.
Further, Japanese Patent Laid-Open No. 2010-026434 (Patent Document 2) discloses an image pickup lens having a five-lens configuration composed of, in order from an object side, a positive first lens, a positive second lens, a negative third lens, a positive fourth lens, and a negative fifth lens.
According to the image pickup lens disclosed in Patent Document 1, the power of the first lens is weak, and the configuration is disadvantageous in achieving thinning of the image pickup lens. The ratio of diagonal length of the image pickup lens with respect to the total track length exceeds 1.0, which makes it difficult to satisfactorily correspond to imaging devices where demands for thinning are increasing. Further according to the disclosed image pickup lens, the F-number is approximately 3.0, which does not exactly ensure sufficient brightness corresponding to imaging elements having increasing number of pixels. As for the image pickup lens disclosed in Patent Document 2, the F-number is as bright as 2.05 to 2.5, and the lens system achieves a high aberration correction ability. However, the power of the first lens is weak, and therefore the configuration is disadvantageous in realizing thinning. The ratio of diagonal length of the imaging element with respect to the total track length is approximately 1.1, which makes it difficult to satisfactorily correspond to imaging devices having increasing demands for thinning, similar to Patent Document 1.